File for a Fictitious Business Name
Unless you intend to receive all incoming payments under your real, legal name (as, say, with a personal consultancy) you need a fictitious business name for your company, also known as a DBA ("doing business as"). You need one because of your bank's policies: If you receive a check for Acme Widgets, you won't be able to cash it unless you can prove that Acme is really you.
To make that connection, get a DBA. This is one of the few actions described in this article that you often can't do on the Web. The specifics of obtaining a fictitious business name vary from city to city and county to county, so you'll need to check with your municipality. In my city, you must make filings in person at the city hall (after you've ensured that no other businesses have the same name), and you must place a notice in a paper of record indicating that you've opened up shop. In some cities--Little Rock, Arkansas, for example--you can do the whole thing online. Some regions require county filings, too.
Check with your official city and county Web sites for specific instructions. Fees will range from nothing to about $50 to have any DBA and relevant licensing (see below) taken care of. Just make certain you go directly to the municipality to do the task: Intermediaries claiming to file forms on your behalf are often expensive scams.
What About Additional Licenses?
Again, this is a locale-specific issue. Some cities make you file for a special license if you're going to be working from home (the city doesn't want you snarling traffic if you open a cookie shop in your kitchen, for example). Others require certain types of businesses to file additional paperwork to get a license. Again, the rules vary dramatically from place to place, but usually you can take care of it all while you're filing for a DBA (and, in fact, usually the city won't give you a DBA unless you've handled any other relevant licensing issues already).
Also, if you're selling physical goods, you'll have to collect sales taxes if your state requires it (as most do). Check your state's Web site to learn about collection and filing procedures. Usually you won't have to pay any up-front fee.